There are many ideas, the most common are below:

•Ae: in the word 'mahfaeraak' the letters 'ae' can be found. This is the only instance of it appearing

•C: There is no 'c' in the dragon language, they may have been trying to replace it with another letter

•Scrapped rune: They may have attempted to make a rune and then scrapped it and kept it in there

These are but some of the ideas surrounding the mysterious 35th rune; sadly the maker of the alphabet died and has never explained the 35th rune, making it a permanently unsolved mystery

Here's a thread about it. It was probably a rune that stood for some letter combination like "ey" or "ah," but ended up being removed.

It's the 35th "unknown rune". It was mapped to 0.